1. Field of the Invention
The present invention relates to seawater-based delayed cross-linking fracturing fluids and methods of using the fluids in fracturing subterranean formations penetrated by well bores.
2. Description of the Prior Art
Oil and gas wells often undergo hydraulic fracturing operations to increase the flow of oil and natural gas from subterranean formations. Hydraulic fracturing is accomplished by injecting a viscous fracturing fluid through the well bore into a subterranean formation to be fractured and applying sufficient fluid pressure on the formation to cause the production of one or more fractures therein. In offshore operations, the fracturing fluid can be prepared utilizing seawater to hydrate a gelling agent to form a viscous aqueous fluid. To promote adequate viscosity at increased well depths, cross-linking agents such as borate ion releasing compounds can be incorporated into the fracturing fluids.
Seawater-based borate cross-linked fracturing fluids perform satisfactorily in low temperature fracturing applications, typically about 200xc2x0 F. or less. At these lower temperatures, the initial pH required to form a sufficiently cross-linked gel is about 9.5 or less. In general, a sufficiently cross-linked gel is defined as having a viscosity of about 200 centipoises or greater at 40/sec shear rate. In order to form a sufficiently cross-linked gel for use at formation temperatures in excess of 200xc2x0 F., the initial pH of a borate cross-linked fracturing fluid must be greater than about 9.5. Elevation of the fracturing fluid pH to a level in excess of 9.5, however, poses several operational problems. For instance, seawater contains multivalent ions such as calcium and magnesium ions which form insoluble precipitates at a pH greater than about 9.5. The presence of solid precipitates reduces the proppant pack conductivity, and ultimately the productivity of the fracturing operation.
Furthermore, elevating the pH of the fracturing fluid to a pH greater than about 9.5 is difficult due to the formation of magnesium hydroxide. Hydroxyl ions needed to elevate the pH of the seawater are instead consumed in the formation of magnesium hydroxide. This reaction proceeds very slowly causing the pH change to be time-delayed and difficult to adjust. In addressing the problems associated with precipitate formation in high-temperature seawater-based fracturing fluids, prior art methods suggest the removal of solid precipitates by filtration. However, the proper disposal of magnesium hydroxide and other precipitates creates additional operational costs as well as environmental challenges.
In order to conduct fracturing operations at greater well depths, it is desirable to delay cross-linking of the fracturing fluid. In particular, a delayed cross-link is advantageous in the fracturing of offshore formations where such operations are usually performed at lower injection rates because of pumping equipment limitations. Reduced injection rates, typically about 10 barrels/minute or less, lead to increased pipe times. Pipe time refers to the time required for the fracturing fluid to make the transit from surface pumping equipment to the formation to be fractured. It is generally desirable to have cross-linking occur near the end of the pipe time as the fluid approaches the formation to be fractured. If cross-linking occurs too early, the increase in fracturing fluid viscosity will increase friction loss in the well bore and produce high pump pressures. To overcome these problems, the cross-linking of the fracturing fluid is delayed until the fluid approaches a location near or within the formation to be fractured.
For these reasons, there is a need for a seawater-based fracturing fluid that avoids precipitate formation and forms delayed cross-links in high temperature fracturing operations.
The present invention provides cross-linked fracturing fluids and methods of using the fluids to fracture subterranean formations which meet the needs described above and overcome the deficiencies of the prior art. The inventive fracturing fluids and methods are particularly useful for use in connection with offshore oil and gas wells. For example, the fluids are stable at temperatures above 200xc2x0 F. and are fully functional at pH levels below the threshold for precipitate formation in seawater, e.g., at a pH of 9.5 or less. Even though the fluids are seawater based, cross-linking can be delayed and controlled in order to facilitate injection of the fluid and other aspects of the fracturing operation.
In one embodiment, the inventive composition is a high temperature delayed cross-linked fracturing fluid comprising:
a gelling agent;
seawater, present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and
a cross-linking agent, capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, whereby the delay in cross-linking is about 5 minutes or more.
In another embodiment, the inventive composition is a high temperature delayed cross-linked fracturing fluid comprising:
a gelling agent;
seawater, present in at least an amount sufficient to hydrate said gelling agent, thereby forming a gelled aqueous fluid; and
a cross-linking agent capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, said cross-linking agent being selected from the group consisting of ammonium titanyl citrate, ammonium titanyl tartarate, ammonium titanyl gluconate, and mixtures thereof.
In yet another embodiment, the inventive composition is a high temperature delayed cross-linked fracturing fluid comprising:
a gelling agent; and
seawater, present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and
a cross-linking agent capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, said cross-linking agent being ammonium titanyl citrate.
In one embodiment, the inventive method of fracturing a subterranean formation penetrated by a well bore and having a temperature above about 200xc2x0 F. basically comprises the following steps:
(a) preparing a seawater-based delayed cross-linked fracturing fluid comprising a gelling agent; seawater present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and a delayed cross-linking agent capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less, whereby the delay in the cross-linking is about 5 minutes or more; and
(b) introducing said fracturing fluid into a subterranean formation at a rate and pressure whereby fractures are formed in the subterranean formation.
In addition to fracturing subterranean formations, the inventive fracturing fluids are useful in connection with other operations as well. For example, the fluids can be used in combination fracturing/gravel packing operations.
A primary advantage of the cross-linked fracturing fluids of the instant invention is that the fluids can be prepared with seawater pumped from the ocean at the site of the fracturing operation, wherever the job happens to be. As a result, the present compositions are inexpensive and simple to prepare, using either batch mixing or on-the-fly procedures.
Another primary advantage is that the inventive cross-linked fracturing fluids are stable at temperatures above about 200xc2x0F. and at a pH of about 9.5 or less. Due to the lower pH, the fluids are compatible with enzyme breakers, and calcium and magnesium salts remain in solution. Even where the gelling agent has been hydrated with seawater, the fracturing fluid provides a delay in cross-linking, conducive to fracturing subterranean formations at greater depths and/or with lower injection rates. Thus, the fracturing fluid has an initial viscosity that is high enough to transport proppant but is not so high as to make pumping difficult.
In general, the cross-linked fracturing fluids of the present invention comprise a gelling agent; seawater present in at least an amount sufficient to hydrate the gelling agent, thereby forming a gelled aqueous fluid; and a cross-linking agent capable of causing delayed cross-linking of said gelling agent at a pH of less than the threshold for precipitate formation in seawater e.g., at a pH of 9.5 or less.
Suitable gelling agents include galactomannan gums, modified or derivative galactomannan gums, and cellulose derivatives. Additional examples of gelling agents which may be utilized in the present invention include but are not limited to guar, hydroxypropylguar, carboxymethylhydroxypropylguar, carboxymethylguar, caboxymethylcellulose, carboxymethylhydroxy-ethylcellulose, and mixtures thereof. Preferred gelling agents include guar and hydroxypropyl guar. Other natural or synthetic polymers which are known in the art but which are not specifically mentioned herein can also be utilized.
The gelling agent is present in the fracturing fluid in an amount in the range of from about 15 to about 60 pounds per 1000 gallons of seawater, preferably from about 20 to about 45 pounds per 1000 gallons of seawater, and most preferably from about 25 to about 35 pounds per 1000 gallons of seawater. Seawater is present in the fracturing fluid in at least an amount sufficient to hydrate the gelling agent.
The cross-linking agent utilized in the present invention is capable of causing delayed cross-linking of the gelling agent for pipe transit times greater than 5 minutes. Thus, the delay in cross-linking exhibited by compositions of the present invention is about 5 minutes or more. Suitable delayed cross-linking agents include but are not limited to ammonium titanyl citrate, ammonium titanyl tartarate, ammonium titanyl gluconate, and mixtures thereof. Of these, ammonium titanyl citrate, the coordination compound ammonium dicitratotitanate, is preferred. The delayed cross-linking agent utilized is in the form of a concentrate having a concentration equivalent to 5.6% TiO2. The delayed cross-linking agent is generally combined with the gelled aqueous fluid in an amount sufficient to provide a titanium concentration in the range of from about 0.45 to about 1.8 percent by weight of said gelling agent.
Propping agents may also be added to the fracturing fluids of the instant invention to hold the fractures open after the fracturing fluid flows back into the well. In general, proppants must have sufficient compressive strength to resist crushing but also must be sufficiently non-abrasive and non-angular to preclude cutting and imbedding into the formation. Examples of suitable proppants include but are not limited to sand, graded gravel, glass beads, sintered bauxite, resin coated sintered bauxite, resin coated sand, ceramics, and resin coated ceramics. Proppants may be present in the inventive composition in an amount in the range of from about 0 to about 25 pounds per gallon, preferably in an amount in the range of from about 0.5 to about 18 pounds per gallon, and most preferably in an amount in the range of from about 0.5 to about 12 pounds per gallon.
The fracturing fluids of the present invention can also comprise a gel breaker which xe2x80x9cbreaksxe2x80x9d or diminishes the viscosity of the fracturing fluid so that it is more easily recovered from the fracture during clean up. Examples of breakers suitable for use with the inventive fracturing fluids include oxidizing agents, enzymes, and acids, with oxidizing agents being the most preferred. The gel breaker is generally present in the fracturing fluid in an amount in the range of from about 0 to about 20 pounds per 1000 gallons, preferably in the range of from about 5 to about 15 pounds per 1000 gallons of seawater, and most preferably in the range of from 5 about 5 to about 10 pounds per 1000 gallons of seawater.
A preferred embodiment of the inventive cross-linking fracturing fluid of the invention comprises a seawater-based solution of hydroxypropyl guar and ammonium titanyl citrate.
The fracturing fluid can include a variety of other conventional additives such as gel stabilizers, clay stabilizers, bactericides, fluid loss additives and the like which do not adversely react with the fracturing fluids or prevent their loss in a desired manner.
The cross-linked fracturing fluids of the instant invention can be prepared by dissolving a gelling agent in seawater to form a gelled aqueous fluid, and then combining with the gelled aqueous fluid a delayed cross-linking agent, capable of causing delayed cross-linking of the gelling agent at a pH of about 9.5 or less. The gelling agent is added to seawater as either a solid or a liquid gel concentrate in a pre-hydrated or slurried form using conventional mixing and pumping equipment. Thereafter, the delayed cross-linking composition is combined with the gelled aqueous fluid. As is understood by those skilled in the art, the cross-linker may be pumped and metered into the gelled aqueous fluid as the gelled aqueous fluid is pumped into the well bore.
The present invention also provides for a method of fracturing a subterranean formation penetrated by a well bore and having a temperature above about 200xc2x0 F., comprising the steps of (a) preparing a delayed cross-linked fracturing fluid comprising a gelling agent, seawater, present in an amount sufficient to hydrate said gelling agent, thereby forming a gelled aqueous fluid; and a delayed cross-linking agent, capable of causing delayed cross-linking of the gelling agent at a pH of 9.5 or less; and (b) introducing the fracturing fluid into a subterranean formation at a rate and pressure whereby fractures are formed in the subterranean formation.
Another method of fracturing subterranean formations with a temperature of more than about 200xc2x0 F. comprises the following steps: (a) preparing a delayed cross-linked fracturing fluid comprising gelling agent; seawater; and a delayed cross-linking agent, said cross-linking agent being selected from the group consisting of ammonium titanyl citrate, ammonium titanyl tartarate, ammonium titanyl gluconate, and mixtures thereof; and (b) introducing said fracturing fluid into a subterranean formation at a rate and pressure whereby fractures are formed in the subterranean formation. The fracturing fluid of the present invention is particularly useful in combination fracturing/gravel packing operations.